How important is a compressor to an air conditioning system? Greg McAfee thinks of it as the rock of the entire operation.

“It’s just like our heart,” said McAfee, president of McAfee Heating and Air Conditioning, Kettering, Ohio. “If it’s got the right pressures and everything like that, it’s going to last longer.”

With the compressor being such a critical element to a system, proper maintenance and repair becomes crucial to compressor and system longevity. But, equally important is correctly diagnosing the problem, ensuring that the problem lies with the compressor.

Randy Tebbe, application engineering manager at Emerson Climate Technologies, said he has had the opportunity to work with the warranty group at Emerson, which has given him a firsthand look at what’s defective and what’s not.

“Approximately one-third of all compressors returned within warranty showed no defect or reason for failure when inspected,” he said. “Compressors typically have an internal or external motor protector designed to take the compressor offline due to high current or temperature. Many times, the high currents or temperatures are system related and the motor protector is only doing its job to protect the compressor. The root cause of the failure can be corrected by the service technician as they perform maintenance on the system. For instance, the on-site technician can correct the system charge, verify the expansion devices, replace external start components, clean coils to unobstruct airflow, repair loose wire connections, and so on.”

That’s why Russ Donnici, president, Mechanical Air Service Inc., San Jose, Calif., always has his service technicians make sure it’s the compressor that’s failed before going forward with repairs on a system.

“From a repair perspective, the No. 1 most important thing is you have to know what caused the compressor to fail previously,” Donnici said. “If you don’t, you’re setting yourself up for a repeat failure. It might not happen immediately, it might happen a year down the road when it’s not on warranty or something like that, but compressors typically do not just fail. There’s usually some external circumstance contributing to the failure. Knowing what caused the original failure is the No. 1 most important thing, because your repair is going to be dependent on that type of knowledge.”

Keeping it Clean

Keeping the system clean is a critical element to maintaining long compressor life, especially when it comes to system controls and sensors, said David Hawkes, director of engineering, North America, at Tecumseh Products Co.

“Typically the biggest failure modes for most compressor technologies are a different part of the system, whether it’s dirt in coils, or loss of refrigerant charge,” Hawkes said. “If we did a better job of sensing when to turn the compressor off, many would probably run many years beyond the design life of the system.”

Another reason compressors fail is due to incorrect filter usage, both by the homeowner and the contractor. That’s why it is crucial to not only use the correct filters, but also to apply them properly.

“A lot of contractors don’t understand filters, sizes of filters, or duct-work size,” McAfee said. “All that has to do with the compressor. If you block airflow, and a lot of times a contractor will sell a real high-efficiency filter when the ductwork can’t handle it, it reduces cfm and puts stress on the compressor.”

Because filter usage ties in so much with airflow, using the correct ones makes the system run much more efficiently, which in turn lessens the load and burden on the compressor.

“Airflow across the coils is something that is easy to maintain, but also easy to overlook,” Tebbe said. “Poor airflow or obstructed airflow will cause the compressor to work harder than it has to work and could lead to premature compressor failure.”

And having good airflow is important for making sure the refrigerant operates as it’s supposed to, said Chris Bryant, lead technician at McAfee Heating and Air Conditioning.

“The minute you don’t have good airflow, the refrigerant doesn’t change state from a liquid to a gas properly and that can lead to issues with the compressor,” Bryant said. “If you’ve got too much liquid going into the compressor it washes the oil out, and if there’s no oil in the bearings, they seize up. If you have too little refrigerant going back to a compressor, refrigerant keeps the compressor cool just like an automobile engine. If a compressor runs overheated, it will actually destroy the electrical motor in the compressor. Short it out and it will die. Everything ties together really intricately.”

In addition to making sure the filters are clean, Donnici also recommends a yearly acid-moisture test on the oil in the compressor and an examination of the superheat, to ensure its correct, which Tebbe agrees is one of the most overlooked aspects of compressor maintenance.

“Too low or no superheat will allow liquid refrigerant to return to the compressor causing a dilution of the refrigerant oil,” Tebbe said. “When liquid refrigerant migrates to the oil, it will reduce the thickness of the oil film which is used by design to protect load-bearing surfaces. Without an adequate oil film, abrasive wear patterns within the compressor will begin to appear. This type of wear is typically an accumulative effect. If the situation moves from oil dilution to liquid slugging, the valves on reciprocating compressors could also be damaged. On the other end of the spectrum, a system with too high of a superheat could cause inadequate motor cooling and high discharge gas temperatures, which could lead to trips on the thermal overload or nuisance trips on discharge line thermostats if applied.”

So what’s the best way to keep a compressor in fine working form? Is it finding out if it even failed? Or is it making sure air is flowing through it properly? What about the superheat settings? Or does it start with the person making the repairs?

“I don’t want to be too hard on our industry, but from a maintenance perspective the No. 1 weakest link is the service tech, whether he’s untrained or lazy,” Donnici said. “From a repair perspective, the No. 1 weakest link is the service tech again. Either he doesn’t find out what the original problem was, or he doesn’t do proper commissioning of the compressor when it’s replaced. That’s why when you have systems that have compressor failure, it’s not unusual for them to have another one because proper procedures were not utilized when they were replaced. Human intervention is the weakest link in any air conditioning system, unfortunately.”

Publication date: 4/1/2013

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